1. Field of the Invention
This invention relates to a photoelectric switch having a light projecting part and a light receiving part and more particularly to a method and apparatus for aligning a light beam from the light projecting part with an optical axis of the light receiving part.
2. Description of the Prior Art
A photoelectric switch detects whether or not an object is presentat a position intercepting a beam of projected light according to whether or not the beam of projected light from a light projecting part is received by a light receiving part. It is necessary to adjust an optical axis so that the beam of projected light from the light projecting part is centered on the light receiving part. Adjustment of the optical axis aided by apparatus for deciding whether or not a beam of projected light is incident correctly on the light receiving part.
A prior art apparatus of this kind is disclosed in Japanese Utility Model Publication No. 25185/1978. The apparatus amplifies an electrical signal generated by a light receiving part as a result of receiving a beam of projected light from a light projecting part. A light emission indicator on the light projecting part and light receiving part is energized by the amplified electrical signal.
The aforementioned apparatus operates the light emission indicator as an indication of the beam of projected light being incident on the lihgt receiving part. However, the light receiving part detects the light beam and generates an electrical signal even when the optical axis of the light beam is somewhat displaced from the axis of the light receiving part. Therefore operation of the light emission indicator, by itself, is not sufficient to discriminate whether the optical axis is disposed in the optimum position or is somewhat displaced.
Another prior art apparatus, disclosed in Japanese Utility Model Publication No. 23243/1973, uses a tuning indicator tube receiving an amplified light reception signal generated by the light receiving prt on reception of a beam of projected light. When the light beam is received on an optical axis at the center of light receiving part, a maximum light reception signal is recognizeable by a minimization of a sector projected on an indicating part of the tuning indicator tube.
However, it is difficult to discriminate an indication on the tuning indicator tube from a distance. This is particularly true when a long distance separates the light emitting part and light receiving part. Thus an indication on the tuning indicator tube at the light receiving part cannot be discriminated from the light emitting part while the optical axis is adjusted. Thus it is difficult to carry out the adjustment.
It is a problem inherent in both prior art techniques that neither can discriminate whether a light beam is incident directly on an optical axis or is displaced therefrom. Also, it is difficult to discern the incident state of the optical axis from the light projecting part side where such adjustment usually is made.